The short answer
Yes, for most ages. The valley runs about 1 hour 15 minutes and 60 km south of Marrakech to Setti Fatma, cooler than the city, with a river to paddle in, riverside cafés with tables set in the stream and the Setti Fatma waterfalls. The walk to the first falls is a guided scramble over rocks, about an hour each way, so it suits older children best.
In more detail
The Ourika Valley is the quickest escape from the city heat. The road climbs from the Marrakech plain into the High Atlas in a little over an hour, following a clear mountain river past mud-brick Berber villages, terraced fields and fruit stalls.
Our Ourika day is built with plenty of short stops. At a women's argan cooperative, children can watch argan nuts pressed by hand and taste amlou, an almond and argan spread. A visit to a Berber family home and mint tea on a terrace above the river follow. Lunch at a table set over the water is an optional add-on.
Setti Fatma is the part to plan. The lower falls are easy to reach, but the walk up to the first cascade is a guided scramble over rocks, roughly an hour each way, and the rocks can be wet. Older children usually love it; with toddlers, stay by the river. Trainers or hiking shoes with grip are a must.
Bring a layer, since it is cooler in the mountains, plus sun hats and sun cream. The day runs from a morning riad pickup to a late-afternoon return.